The technique of eXcess eNvelope enhancement (XNN®) for power amplifiers (PAs), particular for the low power WiFi and WiMAX market applications is disclosed in U.S. Pat. No. 6,437,641. This technique is a simplification of Envelope Tracking (ET), which is disclosed in US 2004/0018821.
The Voltage Enhancement Circuit (VEC™), disclosed in U.S. Pat. No. 6,831,519, modulates the supply voltage of power amplifiers as part of the XNN® technique. Appropriate modulation of the supply voltage prevents saturation of the power amplifier, while amplifying signals that exceed a pre-defined programmable threshold.
Most of the XNN® implementations produce two positive effects while maintaining a constant EVM (Error Vector Magnitude), i.e., increasing the efficiency, while increasing output power or increasing efficiency while reducing DC power supply voltage. In some applications, only efficiency improvement is necessary while the amplifier gain, output power, operating voltage and EVM performance must not be altered. This is effectively results in current reduction of the amplifier, while keeping all other operating parameters constant.
Conventional RF amplifiers required to simultaneously amplify RF signals that have large peak-to-average ratios, are costly and relatively inefficient (i.e., consume much DC power). However, during most of the time, the power output is only a small fraction of the power consumed from the Direct Current (DC) power supply, resulting in low efficiency. The reason for such inefficiency is that on one hand, the DC conditions should be set to values that will be able to provide large power output, but on the other hand, for these DC conditions, such a power amplifier becomes efficient only during the occurrence of the peaks, i.e., when the instantaneous power output is large.
One conventional way to handle signals with large peak-to-average ratios is to control the DC power supply to a power amplifier. One voltage level is supplied to such a power amplifier whenever the instantaneous amplitude is below a given level (normal operating condition), and an enhanced (and higher) voltage level whenever the instantaneous amplitude is above this level. This solution is described for example, in U.S. Pat. No. 6,437,641, which discloses circuitry for dynamically enhancing the operating voltage of an RF amplifier. The resulting input drive to the PA is increased, thereby pushing its output well into saturation and handles the signal clipping by instantaneously enhancing the power supply voltage.
Still, the methods described above has provided satisfactory solutions to the problem of improving the efficiency of power amplifiers operated under large peak-to-average ratios, while eliminating the need for clipping signals having large peak amplitudes.
It is an object of the present invention to provide a method for efficiently controlling the impedance at the output RF path for reducing current consumption of the PA.
It is another object of the present invention to provide a method for efficiently controlling the impedance at the output RF path for increasing the RF large signal gain of the amplifier without increasing the current consumption of the PA.
It is a further object of the present invention to provide circuitry for efficiently and independently controlling the current consumption, gain and output power of the PA.
Other objects and advantages of the invention will become apparent as the description proceeds.